S.M. Rokade, P.M. Bhate/Carbohydrate Research 416 (2015) 21–23
23
Table 3
1.2. General procedure for the preparation of per-O-acetylated
hemiacetals
Recyclability of deep eutectic solvent
Sr. No
Number of runs
Yield %
Amount of DES obtained, g
Acetic anhydride (8.5 g) was added to a solution of the free sugar
(1a–i) (3 g) in DES (15 g) made20 from choline chloride (6.0 g) and
ZnCl2 (9 g), and the resulting mixture was stirred at 90 °C for 1.5 h.
The temperature was raised to 100 °C and stirring continued for
4–6 h. The reaction mixture was cooled to ambient temperature and
extracted with ethyl acetate (3 × 20 mL). The combined ethyl acetate
layer was washed with saturated aqueous NaHCO3, dried over an-
hydrous sodium sulphate and evaporated under vacuum to afford
the corresponding per-O-acetylated hemiacetal.
1
2
3
4
5
Fresh
First
Second
Third
Fourth
88
88
86
85
82
–
15
15.08
15.20
15.43
catalytic quantity of DES.a This result demonstrates the dual role
of DES as a source of acidity as well as an enabling solvent.
We next applied this technique to d-galactose, d-mannose,
d-xylose, l-arabinose, d-fructose, d-ribose, l-rhamnose,
d-glucosamine and d-lactose. In all cases we obtained the corre-
sponding per-O-acetylated hemiacetals (2a–2i) in 80–88% yield and
in significantly lesser time than that previously reported (see Table 2).
TLC of hemiacetals 2a–i showed a single spot. 1H NMR spectra (see
Supplementary material) also indicated absence of impurities. Thus
further purification is not required.
Acknowledgements
S.M.R. thanks the Council of Scientific and Industrial Research
(CSIR), New Delhi, India for providing a research fellowship. The
authors acknowledge work done in the area of DES by Dr. G. S.
Shankarling and co-workers of Department of Dyestuff Technolo-
gy at Institute of Chemical Technology, Mumbai, India.
The recovery and reuse of DES was studied in the reaction of
d-glucose with acetic anhydride. Residual DES after extraction of
the reaction mass with ethyl acetate (see section 1.2) was treated
with water (5 mL) and the resulting mass was dehydrated under
vacuum on a rotary evaporator at 70 °C for 1 h. The dehydrated DES
was directly used for repeating the reaction. Table 3 presents data
obtained after four such recycles. The results indicate that DES can
be recycled three times without significant loss in activity.
In conclusion, we have developed a clean, efficient, and high
yielding one pot protocol for the preparation of per-O-acetylated
hemiacetals from free sugars by using DES made from choline chlo-
ride and ZnCl2 as the solvent. The hemiacetals so prepared do not
require further purification. DES used can be readily recycled.
Appendix: Supplementary material
Supplementary data to this article can be found online at
References
1. Experimental
1.1. General methods
Starting materials and reagents were purchased from commer-
cial suppliers. Thin-layer chromatography (TLC) was performed on
aluminium plates pre-coated with Merck silica gel, spots were ob-
served by spraying the plates with a solution of 10% (v/v) aqueous
H2SO4 with subsequent heating. 1H NMR spectra were recorded on
a Varian (500, 400 MHz) spectrometer. Chemical shifts are ex-
pressed in parts per million (ppm) with TMS as internal standard.
Coupling constants (J) are given in Hz.
aThe authors thank a referee for suggesting this experiment.